The present invention relates to a method of inhibiting the polymerization of styrene at high temperatures. More particularly, the present invention relates to a method of inhibiting the polymerization of styrene which comprises using a methyl-substituted benzoquinone-4-oxime as a polymerization inhibitor.
A large number of compounds have been hitherto studied as a polymerization inhibitor for styrene. However, because the polymerization behavior of styrene at high temperatures is remarkably different from that at low temperatures, a polymerization inhibitor which is useful at low temperatures often exhibits almost no inhibitory activity at high temperatures, for example, in a temperature range (of about 80.degree. to 130.degree. C.) which is encountered during the distillation of styrene. Furthermore, conventional polymerization inhibitors are generally useful in the presence of dissolved oxygen. However, dissolved oxygen is present in a very small quantity in a distillation columm used during the production of styrene. Accordingly, most of the conventional polymerization inhibitors lose their capability of inhibiting polymerization under such a condition. For example, t-butyl catechol, hydroquinonemonomethyl ether or phenothiazine which possesses a high inhibitory capability at a low temperature range exhibits little polymerization inhibiting activity at a high temperature range encountered during the distillation of styrene.
Under these circumstances, in order to inhibit the polymerization of styrene in a distillation column in the final refining distillation step of a process for producing styrene from the dehydrogenation of ethylbenzene, sulfur has hitherto been used. However, since the polymerization inhibiting action of sulfur is not always satisfactory, a large amount of distilled residue called styrene bottom is formed. The styrene bottom contains sulfur, and thus, it poses a problem from the air pollution point of view when it is burned for disposal.
The sulfur contained in the styrene bottom can be recovered by washing with an organic solvent for reuse. This necessitates the provision of an additional recovery apparatus. Accordingly, investigations have been directed to the development of a polymerization inhibitor which can serve as a substitute for sulfur without its drawbacks. Thus, various propositions have been made, for example, in Japanese Patent Publication Nos. 86826/73, 75541/74, 72219/74, 124001/74, 81325/74 and 66687/74. However, these conventional polymerization inhibitors have poor inhibiting activity, are sublimable, explosive, thermally unstable, or prone to produce NO, or have low solubility. Accordingly, these inhibitors are not always satisfactory.
In the case of polymerization inhibition in a styrene distillation column, a polymerization inhibitor should have high solubility in addition to high inhibitory activity. A high solubility of a polymerization inhibitor not only greatly contributes to the development of the activity thereof but also makes it possible to feed continuously a constant flow rate of the inhibitor dissolved in styrene or ethylbenzene. In this respect, p-nitrosophenol (British Patent No. 1064845) which is one of the conventional polymerization inhibitors having the highest inhibiting activity is difficult to feed continuously into a practical distillation column at a constant rate because it has remarkably low solubility in styrene and ethylbenzene.